Fiber slings are widely used as members being used in place of conventional ropes, wires, and rope slings in lifting up and down operations of heavy loads by means of a crane and the like.
Known as fiber slings having a typical structure are round slings having a structure such that: a strand such as prepared from filaments made of high-strength fibers or prepared by twisting loosely filaments is circulated into an annular arrangement as a whole in an arranged condition in many rows, and the whole annular structure constituted by the strand is covered with a protective cover made of cloth. Since such round slings are constituted flexibly as a whole, they have advantages such that a soft contact with a heavy load is possible, so that they little hurt the heavy load, and that the round slings can easily be arranged along the profile of the heavy load, and that the round slings themselves can be easily handled or carried because they are comparatively lightweight. As to such round slings, since a strand excellent in the durability to a load applied during the use, that is, load capacity, is arranged in many rows to make these rows share the load with each other, it is possible to exert a high load capacity as a whole.
As to such fiber slings, there is a case where a part of the strand constituting the fiber slings wear down or become broken as a result such as of the use of the fiber slings under a severe environment for a long period of time. If only a part of the strand is damaged, a load capacity of fiber slings does not so much decrease as a whole, so that it is possible to continue to use the fiber slings as they are. Continuing use of such fiber slings becomes impossible when the fiber slings fall into a state where a load capacity of the fiber slings decreases to such a degree that they cannot be used or where there is no sufficient scope for the capacity of the fiber slings.
In this case, however, it is difficult to look into a degree of damage to the strand contained inside the protective cover by an observation from the outside in the aforementioned structure of the fiber slings. It is a very troublesome operation to check on the inside strand after removing the protective cover. Accordingly, such operation at a job site is in lack of practicality. Under such circumstances, it is demanded to provide a technique to easily determine a degree of decrease in performance of fiber slings, namely, to determine to what degree the load capacity of fiber slings has decreased or to what degree the damage has been done to the strand.
Patent document 1 below discloses a technique such that an inspection conductor for energization is disposed inside an endless strand constituting fiber slings wherein electricity is applied across two connection terminals of the inspection conductor drawn out from the opposite ends of an endless strand. In this case, if there is a portion disconnected at a halfway of the endless strand, the inspection conductor is disconnected also, so that the energization is interrupted. Thus, as a result of the energization inspection, it can be known that there is a disconnection in the endless strand, in other words, deterioration in performance of the fiber slings. There is disclosed also a technique such that an optical conductor is used as an inspection conductor, and it is determined whether or not a light reaches from an end of the optical conductor to the opposite end thereof when the light is applied, whereby it can be known whether there is a disconnection in both the optical conductor and the endless strand or not.
Patent document 2 below discloses also a technique such that a strand of an optical fiber material is disposed in parallel to a strand of a load-bearing material both of which are contained in a cylindrical protective cover for a flexible circular sling, and a condition of the sling is inspected in accordance with whether a light is transmittable between the opposite ends of the optical fiber strand which ends are projected from the protective cover.                [Patent Document 1] Japanese Laid-Open Utility Model Application Publication No. 02-108989        [Patent Document 2] Japanese Laid-Open Patent Application Publication No. 10-305987        
According to the above-described conventional techniques to know deterioration in performance of fiber slings, it is difficult to evaluate adequately performance of the fiber slings, for example, to evaluate how much the deterioration in performance of fiber slings is in detail, or to judge whether or not the fiber slings retain performance which is applicable to a practical use.
For instance, in the prior art of patent document 1 above, only one inspection conductor is disposed along the whole of one endless strand disposed through the whole fiber sling. Accordingly, if a certain position of the endless strand is damaged and the inspection conductor at that position is disconnected, energization or transmission of light is interrupted. Thus, only an alternative evaluation can be made as to performance of the fiber sling. Namely, if the inspection conductor is in a conducting state, the fiber sling functions normally and, if the inspection conductor is in a non-conducting state, the fiber sling does not function normally. In this respect, the technique of patent document 2 above is also quite the same.
However, when a strand is arranged in many rows in a fiber sling, there is a case where a total load capacity all over the fiber sling is not so much reduced if only one raw of the strand among a plurality of rows of the strand is damaged, so the fiber sling is sufficiently practicable. Since many arranged rows of the strand exert their load capacity by means of their mutual frictional bearing force, a load capacity due to the remaining rows which are not damaged is sufficiently maintained. When a fiber sling is designed, rows more than are required for providing a necessary load capacity are made to exist to thus allow a sufficient factor for safety.
According to the above-described prior art, it is judged that a fiber sling is in a malfunction state, even if only one place of an endless strand is damaged. Thus, even such a slightly damaged fiber sling must be discarded, although it retains still sufficient practical performance. Therefore, an economical loss is significant.